The present invention relates to a novel methyltolan type nematic compound useful as an electro-optical display material.
The current market for liquid crystal displays is dominated by TN (twisted nematic) cells which fall under the category of field-effect cells. As reported by G. Bauer in Mol. Cryst. Liq. Cryst. 63, 45 (1981), these TN cells produce interference patterns on the cell surface that can impair the appearance of the cell and in order to prevent the occurrence of such patterns, the product of birefringence (.DELTA..sub.n), of the refractive index of liquid crystal material in the cell and the cell thickness, d (.mu.m), must be fixed to a specified value. In commercial liquid display cells, the product .DELTA..sub.n.d is fixed to one of the values 0.5, 1.0, 1.6 and 2.2. Since the value of .DELTA..sub.n.d may be fixed to the specified value, the use of a liquid crystal marerial having a large value of .DELTA..sub.n will lead to a decrease in d. If d is small, the response time of the cell is shortened in accordance with the well known relation .tau..alpha.d.sup.2. Therefore, a liquid crystal material having a large value of .DELTA..sub.n is of vital importance in the fabrication of a liquid crystal display cell that features quick response and the absence of any interference patterns.
Many of the commercially feasible liquid crystal materials available today are usually prepared by mixing a few or more components made of a compound that has a nematic phase at or near room temperature and a compound that has a nematic phase in ranges higher than room temperature. Most of these mixed liquid crystals commercially used today are required to have a nematic phase over the full temperature range of -30.degree. to +65.degree. C. As the application of liquid crystal display cells increases in diversity, it is desired to develop liquid crystal materials of large .DELTA..sub.n that have a nematic phase in an even higher temperature range and to this end, nematic liquid crystal compounds having large .DELTA..sub.n with high nematic to isotropic (N-I) transition temperatures are required.
With a view to meeting this need, the present inventors previously proposed ##STR3## as a nematic liquid crystal compound having a large .DELTA..sub.n and a high N-I transition temperature (Japanese Patent Application (OPI) No. 152427/85 corresponding to Ser. No. 692,570 now abandoned) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, as it turned out, this compound is not high in miscibility with the nematic mixed liquid crystals commonly used today, especially at low temperatures.